Key metrics that Facebook has exaggerated include the weekly and monthly reach of marketers’ posts, which got inflated by 33 percent and 55 percent, respectively, as the site improperly included repeat visitors in its figures.

Elsewhere, Facebook admitted to exaggerating the number of full views that video ads received, as well as time spent by users reading fast-loading “Instant Articles” for publishers including The Post and the Wall Street Journal, both of which are owned by News Corp.

Facebook insisted that the messed-up metrics — which followed the company’s admission in September that it had inflated its reporting of video viewing times to advertisers by as much as 80 percent — didn’t affect billing to publishers and advertisers.

“It’s not difficult to measure views. It’s not difficult to measure engagement. It’s not difficult to measure any ad metric,” said Mike Gamaroff of Sito Mobile, an ad-targeting firm. “For Facebook to over-report these metrics is pretty inexcusable.”

Earlier this month, Facebook shares got slammed after it told investors it expects a “meaningful” slowdown in ad-revenue growth next year as it seeks to avoid saturating users’ News Feeds with marketing posts.

To fix the ad-metric mess, Facebook said Wednesday it will begin allowing third-party firms like comScore and Moat to vet its viewability data for display-ad campaigns, in addition to video campaigns.

The tech giant also said it is working with TV-ratings firm Nielsen to count video views, and that it’s forming a “Measurement Council” of marketers and ad-agency execs to monitor its metrics.

Still, Facebook stopped short of putting all of its ad measurements up for third-party verification — a stubborn refusal that continues to undermine trust in its ad data, critics say.

“It certainly doesn’t look good,” said Mitchell Reichgut of Jun Group, a New York ad firm. “Online advertising has a history of opaque reporting, and this doesn’t help.”

Facebook, which has more than 4 million advertisers, has been growing its ad revenue this year at more than three times the rate of the overall online ad market, according to Cantor Fitzgerald.